BEIRUT (Reuters) - A Syrian rebel alliance agreed to a three-day nationwide ceasefire announced by the Syrian army on Wednesday and the United States voiced hope a more significant truce could be achieved, though fighting and air attacks continued.

The truce was the first to be declared across the whole country since one brokered by foreign powers in February to facilitate talks to end the five-year-old civil war. That truce has mostly unraveled, and the escalating violence caused talks to break down.

Wednesday's ceasefire covers the three-day Eid al-Fitr holiday celebrated by Muslims to mark the end of the fasting month of Ramadan. But opposition groups and a monitoring organization said little had actually changed on the ground.

"The regime announced the ceasefire, but they did not commit to it. There has been a lot of shelling and bombing on Douma and Daraya (rebel-held towns near Damascus)," a spokeswoman for the Syrian opposition delegation to Geneva peace talks said.

Syria's military high command said in a statement that "a regime of calm will be implemented across all territory of the Syrian Arab Republic for a period of 72 hours from 1 a.m. on July 6 until 2400 on July 8, 2016".

The Syrian government uses the term "regime of calm" to denote a temporary ceasefire.

The Free Syrian Army (FSA) rebel alliance later said it would respect the Eid holiday ceasefire, but only if government forces also abided by it.

"We, the armed revolutionary groups in Syria, welcome any effort towards a ceasefire for the happy Eid al-Fitr period. We declare we will abide by it so long as the other side does the same," an FSA statement said.

"Until now, (the government) has not abided by what it has announced, in that it has launched a number of attacks in various areas today," the statement added.

It said the rebel bloc welcomed international efforts that had yielded the announcement from the government of Syrian President Bashar al-Assad, but attacks had not ceased as a result.

Islam Alloush, spokesman for the powerful Jaish al Islam rebel faction, said in a telegram message to journalists: "The regime has made this announcement purely to escape international pressure. On the ground, I don't think anything has changed."

Jaish al Islam is represented on the official Syrian opposition High Negotiations Committee (HNC).

Jaish al Islam said in a separate statement that, despite the announced truce, government and allied forces had attacked the town of Maydaa, in the Eastern Ghouta area east of Damascus. Maydaa has been held by Jaish al Islam.

The Syrian Observatory for Human Rights said on Wednesday that government and allied forces had taken almost complete control of Maydaa and that fighting continued. Syrian state media said the army and its allies had taken ground from "terrorists" in the area. The Syrian government describes all groups fighting against it as terrorists.

The Britain-based Observatory, which monitors the Syrian conflict, also said there had been rebel and government shelling in areas around the northern city of Aleppo, and air strikes had hit towns in the northern Aleppo countryside on Wednesday.

The Observatory said five air strikes hit the town of Jisr al-Shughour in rebel-held Idlib province, killing two children who were brothers, and injuring a number of others.

Syrian state media also reported army operations against Islamic State militants across the country on Wednesday.

PEACE EFFORT

U.S. Secretary of State John Kerry welcomed the Syrian army's announcement, adding that discussions were under way to try to extend the truce.

"We are trying very hard to grow these current discussions into a longer-lasting ... enforceable, accountable cessation of hostilities that could change the dynamics on the ground," Kerry told a news conference in Tbilisi, Georgia.

The open-ended February "cessation of hostilities" truce, which was intended to smooth talks to end the war, was agreed with many opposition militias, but did not include the al Qaeda-affiliated Nusra Front or Islamic State.

The truce has mostly collapsed and the Syrian army and the Russian military, which backs Assad, have periodically announced a number of temporary local truces in areas of intense fighting, for example in the city of Aleppo or near the capital Damascus.

But air strikes and fighting have often continued in spite of the declarations.

Farah al Atassi, the opposition spokeswoman, said the HNC, which welcomed the ceasefire initiative, would meet in Riyadh after the Eid holiday ahead of an expected resumption of peace negotiations by U.N. special envoy for Syria Staffan de Mistura.

The opposition has consistently said it wants to see full humanitarian aid access across Syria to lay the ground for talks.

"There have been some good stories of aid reaching some besieged areas, but we would like to see that over all Syria. Probably that will be an opportunity to resume the political negotiation," al Atassi said.

(Reporting by Lisa Barrington in Beirut, Lesley Wroughton and Margarita Antidze in Tbilisi and Omar Fahmy in Cairo; editing by Mark Heinrich)